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Article

Experimental Device for the “Green” Synthesis of Unbranched Aliphatic Esters C4–C8 Using an Audio Frequency Electric Field

by
Ioan-Alexandru Udrea
1,
Alexandra Teodora Lukinich-Gruia
2,*,
Cristina Paul
3,*,
Maria-Alexandra Pricop
1,2,
Mircea Dan
1,
Virgil Păunescu
2,4,
Alexandru Băloi
5,
Călin A. Tatu
2,4,
Nicolae Vaszilcsin
1 and
Valentin L. Ordodi
2,3,4
1
Department of Applied Chemistry and Environmental Engineering and Inorganic Compounds, Faculty of Industrial Chemistry and Environmental Engineering, Politehnica University Timisoara, Vasile Pârvan 6, 300223 Timisoara, Romania
2
Centre for Gene and Cellular Therapies in the Treatment of Cancer—OncoGen, Clinical County Hospital of Timisoara, Liviu Rebreanu Blvd. 156, 300736 Timisoara, Romania
3
Department of Applied Chemistry and Engineering of Organic and Natural Compounds, Faculty of Industrial Chemistry and Environmental Engineering, Politehnica University Timisoara, Vasile Pârvan 6, 300223 Timisoara, Romania
4
Department of Functional Sciences, “Victor Babes” University of Medicine and Pharmacy Timisoara, 300041 Timisoara, Romania
5
Department of Electroenergetics, Faculty of Electrical and Power Engineering, Politehnica University Timisoara, Vasile Pârvan 2, 300223 Timisoara, Romania
*
Authors to whom correspondence should be addressed.
Processes 2024, 12(9), 1891; https://doi.org/10.3390/pr12091891
Submission received: 25 July 2024 / Revised: 23 August 2024 / Accepted: 31 August 2024 / Published: 3 September 2024
(This article belongs to the Section Environmental and Green Processes)
Browse Figures
Versions Notes

Abstract

:
One of the most important reactions in organic synthesis is esterification, and the compounds generated using this process are esters with a wide range of applications in various industries. Numerous approaches have been employed to enhance the ester yield and reaction rate and establish equilibrium in esterification reactions. This study uses a non-catalytic thermal esterification method to obtain unbranched aliphatic esters C4–C8. The effect of an audio frequency electric field instead of a catalyst on the esterification reaction between acetic acid and linear C4–C8 aliphatic alcohols was studied. The main goal of this study was to design and implement a lab-scale device for the synthesis of aliphatic esters in an environmentally sustainable manner using only specific raw materials and an audio frequency electric field at 3 and 6 kHz at 20 °C and 50 °C. A mechanism for the esterification reaction using an audio frequency electric field is also suggested. The proposed experimental device is designed to produce esters in a green and cost-effective manner and could be used on a large scale in the food, cosmetics, and pharmaceutical industries.

1. Introduction

Esters are natural or synthetic compounds used in several industries, such as pharmaceutical, food, and cosmetics, as well in as solvents, biofuels, flavoring agents, preservatives, and personal care product formulations. The use of these organic compounds is attributed to their pleasant and fruity odors [1,2]. Fruits are a source of esters, but their composition may change during maturation; thus, extraction from natural sources typically results in low yields and high costs [3,4]. Extraction from natural sources and chemical syntheses are traditional methods for synthesizing these flavor compounds, which are currently being replaced by enzyme-mediated reactions [5], which means a higher production cost due to the high price of enzymes as well as the necessary equipment [6,7]. Over the past few decades, catalyzed esterification reactions have been chosen as the preferable option over non-catalyzed ones in experimental studies due to their high yield, low temperature, low energy requirements, and cheap and fast reactions. The major drawbacks include issues with handling, environmental toxicity, and the fact that using a chemical catalyst results in esters that are not natural and cannot be used in the food industry [8].
The United Nations (UN) promotes the use of greener extraction methods that aim to reduce waste generation and environmental impacts and also increase economic value [9,10]. Today, in the field of chemistry, all new devices proposed for extraction, synthesis, or analysis should follow the principles of green chemistry, such as preventing waste generation, increasing atom economy, using less hazardous chemical syntheses, designing safer chemicals, and using safer solvents and auxiliaries [11].
When designing an extraction or a biocatalytic process, it is important to consider factors such as costs, biocompatibility, efficiency, and prioritizing a more sustainable future. A comparison of esterification reactions with various techniques revealed that conventional processes require a long reaction time to achieve conversion rates higher than 90% [8]. Until now, studies of extraction techniques have focused mainly on the use of non-conventional ones, such as microwave-assisted extraction (MAE), ultrasound-assisted extraction (UAE), pressurized liquid extraction (PLE), pulsed electric fields (PEFs), and supercritical fluid extraction (SC) [1,8,12].
Physical methods, compared to conventional ones in an aqueous medium, are more effective, simpler, and more economical [8]. Using low temperatures and only raw materials, a non-conventional technology using pulsed electric fields (PEFs) has been developed; this method enhances the chemical and physical stability of the dyes, improves energy efficiency and the yield of ester, and allows for better conservation of thermosensitive components like pigments [2]. PEFs represent a technology that involves a short-term treatment by applying a pulsed electric field to the reaction mass. Its advantages include low costs and the use of rapid, natural processes aided by electricity [2]. The orientation of molecular dipoles in alternative PEFs could increase the probability of effective collisions with the formation of reactive products. Starting from raw materials of natural origin, it was demonstrated that natural esters can be obtained using electricity as a catalyst without a harmful effect. The esterification process in the presence of an electric field has the advantage of using only the necessary reactants such as alcohol and acid. If these are obtained by fermentation, then the resulting ester would be 100% natural, without any trace of synthetic by-products, and could be labeled as “natural” and ready for use in the food industry, as well as in cosmetics and pharmacological products [8,12,13]. This emerging technology avoids the requirement of high temperatures and solvents, reducing environmental impacts and enhancing the energy efficiency of the processes. Moreover, it promotes minimal changes when it comes to nutritional and sensory aspects of the product due to the low temperature and holding time of the reaction [3].
The aim of this study was to develop an innovative device based on a resonant transformer, which is applied for the first time to an esterification process. Using this device, the acid and C4 to C8 unbranched alcohols were exposed to an electric field with audio frequencies of 3 and 6 kHz, respectively. After the esterification reaction, the esters were analyzed using two gas chromatography techniques for qualitative and quantitative identification and confirmation. These esters were selected due to their application in the pharmaceutical, food, and cosmetic industries (Table 1). The proposed device provides these industries with an alternative to current methods that promotes a more efficient, faster, and more environmentally friendly synthesis.

2. Results and Discussions

2.1. The Experimental Device Based on Green Technology

The concept of the experimental device presented here follows one of the principles of green technology given its application in the synthesis of esters using an audio frequency electric field as a catalyst without a solvent and without any other “reactant” except electrons, thereby serving as a “green reactant”. Starting from this point, a ferrite core was utilized since it can saturate at a lower flux density, resulting in lower losses and increased resistivity to the eddy currents than the iron core [24]. Therefore, transformers with ferrite cores have the advantage of a very high permeability and low losses [24,25,26] and the disadvantage of easy saturation (saturation flux density < 0.5 T). A ferrite core suppresses electromagnetic emissions by blocking low-frequency noise and absorbing high-frequency noise to avoid electromagnetic interference [25].

2.2. Synthesis of C4–C8 Acetate Esters Using an Audio Frequency Electric Field

The experimental device presented above was used to obtain unbranched C4–C8 acetate esters using acetic acid and the corresponding unbranched C4–C8 alcohols through an esterification reaction under an audio frequency electric field. This type of method aims to replace chemical or enzymatic synthesis, which are currently the most economical and widely used methods [6,27,28]. Esterification methods commonly involve the use of a catalyst that speeds up reaction rates and decreases reaction times. There are two types of catalysts, such as chemical reagents and enzymatic mixtures [6,28,29]. The most common process is the Fischer esterification reaction, which has the limitations of low conversion rates and high reaction times; moreover, the process involves the addition of an excessive amount of alcohol or the continuous removal of water [30]. Esters produced using aggressive chemical catalysts and high-temperature methods cannot be labeled as “natural” because the catalyst can still be found in purified product. The use of other synthesis methods, such as those involving immobilized enzymes, are very popular due to their ecological process and preference for “green” technology, and the resulting esters are generally classified as “natural” [28,31,32].
This method’s innovation involves using an audio frequency electric field instead of a chemical or enzyme catalyst. Key parameters of the esterification process, including the acid/alcohol ratio, the temperature of the reaction, the reaction time, and the frequency of the audio electric field, are discussed below.
The esterification process used a 1:1 stoichiometric molar ratio [33] because the use of these amounts has been reported to achieve high conversions rate [6,34,35]. Also, using excess alcohol could lead to non-reacted alcohol, and this material waste is difficult to separate and recover [36,37].
The reactions were performed using the proposed device in the presence of an audio frequency electric field (at 3 and 6 kHz) as a catalyst for 24 h at 20 °C and 50 °C, respectively. This decision was based on the fact that esterification in the absence of a catalyst is a slow process, as demonstrated by Pipus et al. with a different reaction system that consisted of a microwave tubular flow reactor at a temperature of 80 °C [12]. Other researchers also promote the use of the electric field to catalyze reactions, showing that an external electric field increases the reaction rate, stabilizes the formed intermediates, and drives the equilibrium exclusively toward the desired product [38].
The current literature reveals several methods for performing esterification stoichiometrically with the reactants used in this study, but using various catalysts (Table 2).

2.3. C4–C8 Ester Analysis Using Gas Chromatography Techniques

The esters were analyzed qualitatively and quantitavely using gas chromatography techniques. A GC-FID system was used to quantitatively analyze esters. Furthermore, GC-MS was used to confirm the identity and mass of the obtained esters by comparing their mass spectra to those in the NIST library. Following the chromatographic analyses, the results are expressed as yields of ester obtained after 24 h at the two frequencies used (3 and 6 kHz) compared to the yields obtained without applying the audio frequency electric field according to the parameters already presented.
A typical total ion current diagram (TIC positive chromatogram) obtained from the GC-MS analysis of the C4-acetate ester is presented in Figure 1a, where it can be seen that the ester was obtained. In Figure 1b, the m/z spectrum is presented.
As it is already known, temperature is an important factor influencing the esterification reactions. More precisely, the use of lower temperatures in the esterification reactions leads to lower yields of ester compared to those obtained when higher temperatures are used. Thus, for the reactions performed at 20 °C, the obtained yields of ester were lower than those performed at 50 °C.
In both cases, when the syntheses were performed at 20 °C and 50 °C, the optimal frequency corresponding to the highest yields was 6 kHz. This can be explained by the fact that the application of an audio frequency electric field is more effective at higher frequencies, which destabilizes the O–H bond from alcohols, increases the efficiency of the esterification reaction, and leads to higher yields of ester at both temperatures.
In the case of the reactions performed at 20 °C (Figure 2) and 3 kHz, the yields of ester varied in the range of 2.5–11.3%, with the highest yield noted for n-octyl acetate. When using 6 kHz, the yields varied in the range of 3.5–12.0%, and the highest yield was also noted for n-octyl acetate.
When the reactions were performed without using an audio frequency electric field, the ester yields were lower than in the case of using an audio frequency electric field, varying in the range of 1.7–10.5%.
Increasing the temperature of reactions to 50 °C (Figure 3), the ester yields obtained at 3 kHz varied in the range of 12.5–24.9%, and the highest yield was noted for n-octyl acetate. When using 6 kHz, the yields varied in the range of 13.4–27.9%, and the highest yield was obtained for n-octyl acetate. When the reactions were performed in the absence of an audio frequency electric field, the yields of ester were lower than those obtained when using an audio frequency electric field, varying in the range of 8.9–17.5%.
Experimental determinations have shown that the audio frequency electric field has a positive impact regarding the esterification reaction of acetic acid with different alcohols; for all reactions, the yields of ester were higher when the audio frequency electric field was applied (regardless of the temperature or frequency used). Due to the orientation of the molecular dipoles in the audio frequency electric field, the probability of effective collisions is increased; thus, higher yields of ester are obtained.

2.4. Mechanism of Esterification Using an Audio Frequency Electric Field

In order to increase the reaction yields, the esterification reaction is generally dependent on a catalyst, typically an acid. Using an audio frequency electric field can have a positive impact on the reaction through the polarization of reactant molecules, reduction of activation energy, and favorable orientation of the molecules, even without a catalyst [43,44]. For the esterification reaction using an audio frequency electric field as a catalyst, we propose the following six steps mechanism illustrated in Figure 4.
When an audio frequency electric field is applied, the reactant molecules are polarized (Figure 4(I)), causing the carboxylic acid to become more electrophilic and the alcohol to become more nucleophilic because their dipole moments align. Thus, polarized alcohol more easily attacks (Figure 4(II)) the carbonyl carbon of polarized carboxylic acid, and a tetrahedral intermediate is formed (Figure 4(III)). In other words, the audio frequency electric field increases the nucleophilicity of the alcohol and the electrophilicity of the carboxylic acid, facilitating nucleophilic attack.
The audio frequency electric field also stabilizes the tetrahedral intermediate that is obtained after nucleophilic attack and reduces the activation energy. Within the tetrahedral intermediary, a proton transfer takes place (Figure 4(IV)), which leads to the formation of a more stable intermediate. The audio frequency electric field plays the role of a charge stabilizer during this transfer. By stabilizing the transition state and reducing the activation energy, the audio frequency electric field helps to remove a molecule of water from the tetrahedral intermediate (Figure 4(V)). The final stage involves ester formation (Figure 4(VI)) and the release of water and one proton. Throughout the process, the audio frequency electric field orients and stabilizes the molecules, which enhances the reaction yield, even in the absence of a catalyst.

2.5. AGREE Analysis

Green chemistry has developed over the years. Since 2000, the “greenness” of the analytical chemistry has been given an important statute. Green chemistry metrics are used by chemists to provide a conclusion about the preparation and the safety of the analytical method. The procedures for determining whether an analytical procedure can be accepted as green should be standardized and employ a valid assessment tool. These procedures must be compared, validated, and incorporated as the main parameter in planning the development of green analytical methods [1,11,45,46]. Among the green assessment tools available, the AGREE tool is a simple automated and reliable tool that provides more detailed information [45]. The AGREE tool has a wider score range that provides sufficient levels of accuracy and specificity in assessing the greenness of the methods studied.
The AGREEprep tool (v. 0.91) [46,47] was utilized to evaluate ester synthesis in the audio frequency electric field, and AGREE analytical GREEnness (v. 0.5 beta) was utilized to evaluate the analytical technique (gas chromatography technique) [45]. The results are output as a colored pictogram (Figure 5). The overall score and color representation in the middle indicate the weak and strong points of the ester synthesis under the current conditions (Figure 5a) as well as the analytical techniques (Figure 5b).
The strong points are noted in light and dark green colors; yellow, orange, and red indicate the weak points. The overall score in the evaluation of ester synthesis in the audio frequency electric field (Figure 5a) is 0.66. The strongest points include the sample preparation, the use of no hazardous materials (the use of an audio frequency electric field instead of a chemical catalyst and no organic solvent is used), the sustainability and reusability of the reagents, the small volumes of waste (based on calculations for the most unfavourable case, 0.5 g of water cannot be reused), low energy consumption (energy is absorbed from the grid and the entire experimental process uses 30 Wh), and operator safety. The weak points of the ester synthesis procedure (Figure 5a) include the volume of the sample collected, the long reaction time (24 h), the use of a manual system without automation, and post-sample configuration for gas chromatographic analysis.
AGREEprep analysis was also performed for the synthesis that occurred in the absence of the audio frequency electric field (Figure S1 in Supplementary Materials). The overall score was slightly higher (0.71) than the one obtained for the synthesis in presence of the audio frequency electric field because no energy was consumed and the volume of waste was reduced (0.34 g of water that cannot be reused). Considering that the yields of ester are lower in the absence of an audio frequency electric field (as shown), the benefits of the presence of the electric field for the synthesis of unbranched aliphatic esters C4–C8 are obvious.
The strongest points of the AGREE analytical GREEness for the gas chromatography technique (Figure 5b) include the direct analysis of a minimal size sample (0.2 mL) without derivatization, automated analysis, multi-analyte identification in a single run (4 analytes/run; 6 samples/h), reduced analytical waste, and operator safety. One of the weakest points in the analytical procedure was the GC-FID method given its energy consumption, the solvent used for the dilution of the samples (acetone not bio-based), and the solvent’s toxicity and flammable characteristics (Figure 5b).

2.6. Strengths, Limitations, and Future Perspectives

The present device follows some principles of green chemistry based on the design of a synthesis method that does not use or produce hazardous materials. This method eliminates the need for acid catalysts or solvents, resulting in a natural, safe, and high-quality final product that can be used directly in the food, pharmaceutical, and cosmetic industries.
The main limitation of this study is the use of a variable in the reaction, such as the audio-frequency catalyst of the esterification process. In addition, improving certain parameters of the device could reduce the long reaction time. To enhance this method, it would be beneficial to utilize other acids with a longer carbon chain, such as fatty acids, and aliphatic unbranched and branched alcohols with a longer carbon chain. Other frequencies of the electric field could be used to increase the esterification rate. Furthermore, the device could have a wireless power transfer system for efficient application [46]. The usage of this device in sustainable technologies, such as wastewater purification, extraction, and oxidation processes, could potentially lead to its future implementation on a large scale.

3. Materials and Methods

The reagents used for these studies are commercial grade and were used without further purification: acetic acid (99%, Chimreactiv, Bucharest, Romania), n-butanol (99%, Merck, Rahway, NJ, USA), n-pentanol (99%, Merck), n-hexanol (99%, Merck), n-heptanol (99%, Merck), n-octanol (99%, Merck), acetone (99%, Sigma-Aldrich, St. Louis, MI, USA), and nitrobenzene (99%, Sigma-Aldrich).

3.1. The Experimental Device

The experimental device proposed in this paper (Figure 6) is a resonant transformer with an open ferrite core. Additionally, the proposed device was made by following the developmental steps outlined schematically in Figure 7.
The reaction vessel (Figure 6e) is a cylindrical condenser with a stainless-steel central electrode and an external copper thermostatic jacket (Figure 6f) as armatures. The dielectric medium consists of the reaction mass and the glass vessel in which the physical–chemical phenomenon occurs. The distance between the two armatures is the radius of this assembly and is 15 mm. The capacitor has 100 pF capacity measured with the LCR-meter 880 BK Precision (B&K PRECISION, Yorba Linda, CA, USA). The inductance of the transformer secondary coil, together with the capacitor capacity, forms a parallel inductor–capacitor network or LC circuit that is tuned to the desired frequency (1–10 kHz) by manipulating the ferrite core [48].
The high voltage (in the range of kV) at the terminals of this circuit is measured with an electrostatic voltmeter (Figure 6f) with a very high impedance and a very low capacity. The voltmeter has a fixed copper electrode against which a small tube with neon discharges is moved. This acts as an optical indicator that begins to emit red–orange light when it is located in an area of the electric field current of sufficient intensity to generate a luminescent discharge. This intensity of the electric field is proportional to the voltage applied to the terminals of the device and is proportional to the distance at which the optical indicator is located from the copper electrode read on the device scale.
The primary coil is built on a separate plastic housing and placed towards the cold end of the secondary coil, thus forming the resonant transformer. This coil is connected in a series with an equivalent capacitor consisting of 19 fixed capacitors (Figure 6d) having a capacity of 0.1 microF, which can be individually inserted into the circuit by manipulating 19 ON–OFF contacts. The value of the working capacity of this equivalent capacitor depends on the number of fixed capacitors connected. A series of LC circuits is thus formed and tuned around the working frequency by changing the capacity of the equivalent capacitor as described above. This oscillating circuit is the load of the power amplifier made with the TDA2003 integrated circuit. The intensity of the audio frequency current is visually estimated by measuring the brightness of the incandescent bulb (Figure 6c) connected in the circuit. The input signal of this amplifier is taken from a sinusoidal signal generator (Figure 6a) with adjustable frequency (1 to 10 kHz) and adjustable amplitude (0 to 1 V).
Operating the device requires tuning of the primary series of LC circuits, aiming to obtain maximum illumination of the incandescent bulb. Then, the secondary parallel LC circuit is tuned to obtain a maximum electric voltage at the electrostatic voltmeter (Figure 6j). Finally, the high voltage is adjusted to 15 kV, which is the value established by the experimental protocol, by changing the amplitude of the audio frequency signal from the generator.

3.2. Synthesis of Aliphatic Esters Using an Audio Frequency Electric Field

Acetic acid and unbranched alcohols (n-butanol, n-pentanol, n-hexanol, n-heptanol, n-octanol) in a 1:1 molar ratio were added for a total reaction volume of 20 mL.
The reactants were mixed in a 30 mL glass reaction vessel (Figure 6e) at 20 °C and 50 °C, atmospheric pressure, and at 3 and 6 kHz frequencies for 24 h. In addition, control samples were defined as reactions that occurred in the absence of the audio frequency electric field. All esterification experiments were performed in duplicate. The 1:1 molar ratio of alcohol and acetic acid was chosen because it represents the equilibrum reaction, or more precisely, the worst reaction condition.
The obtained amounts of ester (mg) in the esterification reactions were monitored with a gas chromatography technique (GC-FID) using acetone as solvent and nitrobenzene as an internal standard (IS). The confirmation of ester formation was performed using the GC-MS method.

3.3. GC-FID Analysis

The reaction mixture was subjected to gas chromatography analyses (GC-FID) on a Varian 450 Chromatograph (Varian Inc., Utrecht, The Netherlands) equipped with a flame ionization detector (FID) using a VF-1MS non-polar capillary column (15 m × 0.25 mm, and 0.25 µm film thickness of dimethylpolysiloxane). The analysis conditions were as follows: oven temperature of 50 (hold 0.5 min)—75 °C (hold 1 min), with a heating rate of 3 °C/min, injector temperature of 300 °C, detector temperature of 350 °C, and carrier gas flow (hydrogen) of 2.0 mL/min. The samples were diluted in acetone, and a quantitative analysis was performed using 25 µL nitrobenzene as an internal standard. The samples analysis were run in triplicate, and the mean values were considered.

3.4. GC-MS Analysis

Ester samples were diluted 1:10 in acetone prior to GC-MS analysis. The structure of the esters was confirmed by GC-MS, with an electron ionization (EI) of 70 eV, on a Hewlett-Packard HP6890 gas chromatograph and an HP5973 mass spectrometer (Agilent Technologies, Santa Clara, CA, USA). Here, 1 µL ester sample was injected in splitless mode on a DB-WAX high-polarity, polyethylene glycol (PEG) capillary column (30 m × 0.25 mm × 0.25 µm film thickness) (Agilent Technologies J&W Scientific, INC, Santa Clara, CA, USA). The analysis conditions were as follows: the oven temperature ranged between 50 and 250 °C with a rate of 5 °C/min, a final hold of 5 min, and a flow rate of 1 mL/min. The MS conditions were as follows: the source temperature was set at 230 °C, and the quadrupole detector temperature was set at 150 °C. The MS detector was set in scan mode, and the mass range of compounds was from 50 to 600 amu and started to register after 3 min of solvent delay. Peaks were integrated with the MS software Chemstation Data Analysis (Agilent Technologies, Santa Clara, CA, USA) and AMDIS (NIST, Gaithersburg, MD, USA).

3.5. Evaluation of Preparation and Analytical Methods Using AGREE Tools

AGREEprep was the first tool designed for the assessment of the greeness of analytical sample preparation. This tool was used to evaluate our ester preparation technique. It considers 10 criteria that cover aspects contributing to the greenness of sample preparation: 1—Sample preparation placement; 2—Use of safer solvents and reagents; 3—Target sustainable, reusable, and renewable materials; 4—Minimize waste; 5—Minimize sample, chemical, and material amounts; 6—Maximize sample throughput; 7—Integrate steps and promote automation; 8—Minimize energy consumption; 9—Choose the greenest possible post-sample preparation configuration for analysis, and 10—Ensure safe procedures for the operator [46]. AGREEprep is an open access software. The evaluation process is effortless, and the intuitive interface of the software makes the interaction process effective for both entering values and reading results [46].
The evaluation of the analytical technique (gas chromatography method) was performed with the AGREE analytical GREEnness calculator, which is based on the 12 significant principles of green analytical chemistry: 1—Direct analytical techniques should be applied to avoid sample treatment; 2—Minimal sample size and minimal number of samples are goals; 3—If possible, measurements should be performed in situ; 4—The integration of analytical processes and operations saves energy and reduces the use of reagents; 5—Automated and miniaturized methods should be selected; 6—Derivatization should be avoided; 7—The generation of a large volume of analytical waste should be avoided, and proper management of analytical waste should be provided; 8—Multianalyte or multiparameter methods are preferred versus methods using one analyte at a time; 9—The use of energy should be minimized; 10—Reagents obtained from renewable source are preferred; 11—Toxic reagents should be eliminated or replaced, and 12—Operator’s safety should be increased [45]. AGREE Analytical GREEnness is a free downloadable software used to assess the greenness of analytical procedures; it is comprehensive, flexible, easy to use and interpret, and both fast and straightforward [45].

4. Conclusions

In this study, five esters of acetic acid with small linear aliphatic alcohols were synthesized in an environmentally sustainable manner, exclusively in the presence of an audio frequency electric field. The yields of ester for all reactions were higher when an audio frequency electric field was applied, regardless of the temperature or frequency, compared to the yields obtained in the absence of an audio frequency electric field.
The proposed laboratory-scale device was first presented for this purpose, and the obtained results can be considered a starting point in the synthesis of “green” materials without catalysts. Therefore, to address economic and environmental issues, the device described here could be a valuable alternative for emerging ‘green’ technologies.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/pr12091891/s1, Figure S1: Results obtained using the AGREEprep tool for the evaluation of ester synthesis in the absence of audio frequency electric field (image created using the open access software AGREEprep available at https://mostwiedzy.pl/en/wojciech-wojnowski,174235-1/agreeprep, accessed on 17 July 2024).

Author Contributions

Conceptualization, I.-A.U. and V.L.O.; methodology, V.L.O.; validation, C.P., A.T.L.-G. and V.L.O.; formal analysis, C.P., A.T.L.-G. and V.L.O.; investigation, I.-A.U., C.P., M.-A.P. and V.L.O.; resources, C.A.T., M.D., N.V., V.P. and V.L.O.; data curation, A.T.L.-G., A.B. and V.L.O.; writing—original draft preparation, A.T.L.-G. and V.L.O.; writing—review and editing, A.T.L.-G., C.P., C.A.T. and V.L.O.; visualization, A.T.L.-G., C.A.T. and V.L.O.; supervision, N.V., V.P., A.B. and V.L.O. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are contained within the article.

Acknowledgments

The authors would like to thank Eng. Ion Burcă for all his support regarding the illustrated mechanism of the esterification reaction.

Conflicts of Interest

The authors declare no conflicts of interest.

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Processes 12 01891 g001
Figure 1. (a) TIC chromatogram of the C4-acetate ester; (b) The m/z spectrum of the C4-acetate ester.
Figure 1. (a) TIC chromatogram of the C4-acetate ester; (b) The m/z spectrum of the C4-acetate ester.
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Processes 12 01891 g002
Figure 2. Influence of the audio frequency electric field (kHz) and the reaction temperature on the ester yield in the esterification reactions performed at 20 °C. (noEF—in the absence of an electric field).
Figure 2. Influence of the audio frequency electric field (kHz) and the reaction temperature on the ester yield in the esterification reactions performed at 20 °C. (noEF—in the absence of an electric field).
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Processes 12 01891 g003
Figure 3. Influence of the audio frequency electric field (kHz) and the reaction temperature on the ester yield in the esterification reactions performed at 50 °C. (noEF—in the absence of an electric field).
Figure 3. Influence of the audio frequency electric field (kHz) and the reaction temperature on the ester yield in the esterification reactions performed at 50 °C. (noEF—in the absence of an electric field).
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Processes 12 01891 g004
Figure 4. The proposed mechanism for the esterification reaction using an audio frequency electric field as a catalyst: step I—polarization of reactants; step II—nucleophilic attack; step III—formation of the tetrahedral intermediate; step IV—proton transfer; step V—dehydration; step VI—formation of ester.
Figure 4. The proposed mechanism for the esterification reaction using an audio frequency electric field as a catalyst: step I—polarization of reactants; step II—nucleophilic attack; step III—formation of the tetrahedral intermediate; step IV—proton transfer; step V—dehydration; step VI—formation of ester.
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Processes 12 01891 g005
Figure 5. (a) Results obtained using the AGREEprep tool for the evaluation of ester synthesis in the audio frequency electric field (image created using the open access software AGREEprep available at https://mostwiedzy.pl/en/wojciech-wojnowski,174235-1/agreeprep, accessed on 17 July 2024); (b) Results obtained using the AGREE analytical GREEnness tool for assessment of the analytical technique (image created using the open access software AGREE analytical GREEnness available at https://mostwiedzy.pl/en/wojciech-wojnowski,174235-1/AGREE, accessed on 17 July 2024).
Figure 5. (a) Results obtained using the AGREEprep tool for the evaluation of ester synthesis in the audio frequency electric field (image created using the open access software AGREEprep available at https://mostwiedzy.pl/en/wojciech-wojnowski,174235-1/agreeprep, accessed on 17 July 2024); (b) Results obtained using the AGREE analytical GREEnness tool for assessment of the analytical technique (image created using the open access software AGREE analytical GREEnness available at https://mostwiedzy.pl/en/wojciech-wojnowski,174235-1/AGREE, accessed on 17 July 2024).
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Processes 12 01891 g006
Figure 6. Device used for the “green” synthesis of unbranched aliphatic esters C4–C8 using an audio-frequency electric field: (a) sinewave generator, (b) ammeter, (c) incandescent bulb, (d) capacitors, (e) glass reaction vessel, (f) copper jacket for cooling/heating, (g) magnetic stirrer, (h) HV coil, (i) connection cables, and (j) electrostatic voltmeter.
Figure 6. Device used for the “green” synthesis of unbranched aliphatic esters C4–C8 using an audio-frequency electric field: (a) sinewave generator, (b) ammeter, (c) incandescent bulb, (d) capacitors, (e) glass reaction vessel, (f) copper jacket for cooling/heating, (g) magnetic stirrer, (h) HV coil, (i) connection cables, and (j) electrostatic voltmeter.
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Processes 12 01891 g007
Figure 7. The proposed schematic representation provides a one-to-one depiction of the device used for “green” synthesis (image created with PowerPointDesigner, Microsoft 365, Microsoft Corporation, Redmond, WA, USA).
Figure 7. The proposed schematic representation provides a one-to-one depiction of the device used for “green” synthesis (image created with PowerPointDesigner, Microsoft 365, Microsoft Corporation, Redmond, WA, USA).
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Table 1. The unbranched aliphatic esters from acetic acid obtained in this study.
Table 1. The unbranched aliphatic esters from acetic acid obtained in this study.
EsterChemical StructureCharacteristics/UsageReferences
n-Butyl acetateProcesses 12 01891 i001Sweet scent of banana or apple employed in various industries, including food, beverage, fragrance, cosmetics, and pharmaceutical industries[14,15,16]
n-Pentyl acetateProcesses 12 01891 i002Scent like bananas and apples used as a flavoring in foods and as a scent in perfumes[17]
n-Hexyl acetateProcesses 12 01891 i003Fruity smelling scent used as a flavoring agent or as a scent in perfumes; exhibits antimicrobial activity and can be used to improve the safety of minimally processed fruits[18,19,20]
n-Heptyl acetateProcesses 12 01891 i004Pear scent/essence used as flavoring in foods and as a scent in perfumes[21,22]
n-Octyl acetateProcesses 12 01891 i005Scent like oranges, grapefruits, and other citrus products used as a flavoring in foods and as a scent in perfumes[23]
Table 2. Examples of optimum conditions for esterification reactions at a 1:1 molar ratio.
Table 2. Examples of optimum conditions for esterification reactions at a 1:1 molar ratio.
ReactantsCatalystTemperature (°C)Reaction Time (min)Yield (%)ProductExperiment SetupReferences
Oleic acid/
Oleyl alcohol		H2SO4
Perchloric acid
Phosphoric acid		9030093.88
54.9
52.7		Oleyl oleateOil bath
stirring		[39]
Acrylic acid/
Ethanol		H2SO47036083.99Ethyl acrylateBath reactor[40]
Oleic acid/
1-Octanol		DBSA (dodecylbenzene sulfonic acid)23144098.7Octyl oleateDean–Stark
apparatus		[34]
Acetic acid/
Ethanol		Ionic liquid (1-(4-sulfonic acid) butylpyridinium hydrogen sulfate)10024099Ethyl acetateOil bath
stirring		[41]
Acetic acid/
n-Hexanol		Sulfonic acid-functionalized MIL-10111030099Hexyl acetateDean–Stark
apparatus		[42]
Octanoic acid/
Hexanol		Candida antarctica lipase (Novozym 435)356090Hexyl octanoateGlass vials and thermoshaker[7]
Benzoic acid/1-PropanolTriphenylphosphine and iodine853093Propyl benzoateMicrowave
reactor		[13]
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Udrea, I.-A.; Lukinich-Gruia, A.T.; Paul, C.; Pricop, M.-A.; Dan, M.; Păunescu, V.; Băloi, A.; Tatu, C.A.; Vaszilcsin, N.; Ordodi, V.L. Experimental Device for the “Green” Synthesis of Unbranched Aliphatic Esters C4–C8 Using an Audio Frequency Electric Field. Processes 2024, 12, 1891. https://doi.org/10.3390/pr12091891

AMA Style

Udrea I-A, Lukinich-Gruia AT, Paul C, Pricop M-A, Dan M, Păunescu V, Băloi A, Tatu CA, Vaszilcsin N, Ordodi VL. Experimental Device for the “Green” Synthesis of Unbranched Aliphatic Esters C4–C8 Using an Audio Frequency Electric Field. Processes. 2024; 12(9):1891. https://doi.org/10.3390/pr12091891

Chicago/Turabian Style

Udrea, Ioan-Alexandru, Alexandra Teodora Lukinich-Gruia, Cristina Paul, Maria-Alexandra Pricop, Mircea Dan, Virgil Păunescu, Alexandru Băloi, Călin A. Tatu, Nicolae Vaszilcsin, and Valentin L. Ordodi. 2024. "Experimental Device for the “Green” Synthesis of Unbranched Aliphatic Esters C4–C8 Using an Audio Frequency Electric Field" Processes 12, no. 9: 1891. https://doi.org/10.3390/pr12091891

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